Defect and Diffusion Forum Vols. 251-252

Abstract: The purpose of this work is to study the effect of bulk point defects on the electronic structure of rutile TiO2. The paper is focused on the effect of oxygen nonstoichiometry in the form of oxygen vacancies, Ti interstitials and Ti vacancies and related defect disorder on the band gap width and on the local energy levels inside the band gap. Ab initio density functional theory is used to calculate the formation energies of such intrinsic defects and to detect the positions of these defect induced energy levels in order to visualize the tendency of forming local mid-gap bands. Apart from the formation energy of the Ti vacancies (where experimental data do not exist) our calculated results of the defect formation energies are in fair agreement with the experimental results and the defect energy levels consistently support the experimental observations. The calculated results indicate that the exact position of defect energy levels depends on the estimated band gap and also the charge state of the point defects of TiO2.

Abstract: Three Zinc Molybdenum ceramic samples doped Bi have been prepared according to the chemical formula (1-x) ZnO – 0.2MoO3 – xBi2O3, where (x = 0.2, 0.5, 1.0) mol %. The samples were studied at room temperature through X-ray Diffraction analysis, SEM, EDAX, I-V characteristics and C-V measurements. The results decleared the presences of two phases; ZnO as a major phase beside Bi2MoO6 as a minor phase. At lower Bi additions; Bi and Mo ions are highly segregated at the grain boundaries, while at higher additions more homogenous distribution for these ions inside the grain is observed. Some pores are observed around the batches of the minor phase and at the grain boundaries. The grain size is enlarged with increasing Bi addition; while the number and size of the pores are decrease. Creation of the pores is attributed to oxygen liberation from the surface of the sample during sintering. Formation of Schottky barrier is indicated via I-V and C-V measurements and attributed to Zn vacancies at the grain boundary. Interface potential barrier, donor density, interface state density and barrier width are decrease with increasing Bi addition. I-V characteristics revealed voltage switching. The switching voltage E0 decreases with increasing Bi addition and it is reproducible even after several on-off cycles.

Abstract: ZrW2O7(OH1-x,Clx)2·2H2O (x = 0.016, 0.025) is prepared by using the acid steam hydrothermal (ASH) method. The chlorine contents of the crystals are measured by means of potentiometric titration and ICP, respectively. The as-prepared powders are characterized by X-ray diffraction, and their structures are refined by using the Rietveld method contained in the GSAS program. The results indicate that the chloride ion occupies the hydroxyl group crystal site statistically.

Abstract: We have investigated deep trap concentrations in hydride vapor pressure epitaxy (HVPE) - grown GaN by measuring three-dimensional carrier concentration profiles and ionization energies. Schottky contacts were fabricated on 28-68μm thick films using Ni/Au contacts. Extensive capacitance-voltage measurements were made in the temperature range 100-350K at reverse bias voltages in the range 0 to –5V. Effective carrier concentrations and ionization energies were determined from three-dimensional plots of concentration-temperature-depth. Carrier concentration versus temperature plots show slowly changing three-step behavior. During the first step, all the plots rise linearly up to about 200K reaching respective plateaus before reversing courses downwards again linearly. Ionization energy plots, on the other hand, are almost linear all the way up to 350 K showing some tendency of upward bending. Trap concentrations were determined from carrier concentrations and previously measured deep level transient spectroscopy (DLTS) plots as function of reverse bias voltages. In almost every case, trap concentrations also rise linearly with increasing depth in the samples.

Abstract: The mechanism of slow positron annihilation in Si-doped GaAs has been discussed in terms of the diffusion trapping model (DTM). The trapping of positrons has been considered in SiAs acceptors i.e. shallow defects and in VGa-SiGa vacancy complexes. The model has been used to obtain the Doppler broadening line shape parameter (S-parameter) and average positron lifetime in Si-doped GaAs, for a temperature range 20K to 290K and for different doping concentrations. Observations are made regarding the effect of doping on the nature and concentration of point defects. The change in point defect concentration due to Si- doping has been found to be proportional to the doping concentration. The effect of detrapping from the shallow defects has been found to be important at higher temperatures.

Abstract: The six-jump-cycle (6JC) mechanism is used to derive expressions for collective correlation factors in a nonstoichiometric binary intermetallic compound AB. The 6JC is used as a fundamental unit for the cycle involving a perfectly ordered configuration and a two-jumpcycle (2JC) as a fundamental unit for the cycle involving existing antistructural atoms. The jump frequency for the 6JC is calculated in terms of a four-frequency-model using the mean first passage concept of Arita et al., while the jump frequency for the 2JC is taken to be the harmonic mean of the individual jump frequencies. The expressions for phenomenological transport coefficients are obtained through the linear response approximation using the kinetic equation approach. The results for collective correlation factors are compared with Monte Carlo simulation and are found to be in reasonably good agreement when the ratio of jump frequencies of regular site and antistructural atoms is of the order of 10-1.

Abstract: The six-jump-cycle (6JC) diffusion mechanism is used to analyze the behavior of vacancy-wind factors and collective correlation factors in partially ordered B2 intermetallic compounds at stoichiometric and near- stoichiometric compositions. Expressions for the vacancywind factors are obtained in the framework of the four-frequency model where the two sublattices exist a priori. The phenomenological coefficients on the two sublattices that remain hitherto independent in 6JC mechanism are connected through a microscopic detailed balance condition. The present results for collective correlation factors when compared with our earlier calculation based on taking the harmonic mean of the sublattice correlation factors show much better agreement with Monte Carlo simulation results. The collective correlation factors and tracer correlation factors are used to calculate the vacancy-wind factors. Our results for vacancy-wind factors agree qualitatively with the simulation data when the frequency ratio ( α ) of structural and antistructural atoms jumps decreases up to the order of unity.

Abstract: Positron Annihilation Doppler Broadening Spectroscopy (PADPS) is one of the nuclear techniques used in material science. PADPS measurements are used to study the behavior of defect concentration and dislocation density in a set of 3003 and 3005 wrought aluminum alloy. It has been shown that positrons can become trapped at imperfect locations in solids. The S-parameter can be influenced by changes in the concentration of such defects. There is no observed change in the Sparameter values after the saturation of defect concentration. The S-parameter and trapping rates for the samples deformed up to 10 percent were studied. The concentration of defect range varies from 1017 to 1018 cm-3 and from 1016 to 1017 cm-3 for 3003 and 3005 wrought Al alloy respectively. While trapping rate range varies from 1 x1010 to 1.2x1011 s-1 for 3003 and from 1 x109 to 1.2x1010 s-1 for 3005 wrought Al alloy.

Abstract: Creep experiments were conducted on Cu-8.5at.% Al alloy in the intermediate temperature range from 673 to 873K, corresponding to 0.46-0.72 Tm where Tm is the absolute melting temperature. The present analysis reveals the presence of two distinct deformation regions (climb and viscous glide) in the plot of log ε vs. log σ. The implications of these results on the transition from powerlaw to exponential creep regime are examined. The results indicated that the rate controlling mechanism for creep is the obstacle-controlled dislocation glide. A phenomenological model is proposed which assumes that cell boundaries with sub-grains act as sources and obstacles to gliding dislocations.